In the fabrication of certain semiconductor devices, such as monolithic infrared image sensors, it is desirable to provide ultra thin metal silicide layers on semiconductor substrates, particularly high resistivity silicon substrates. In order to obtain maximum sensitivity, it is further desirable that such layers be as thin as possible, preferably less than 10 nanometers in thickness and, ideally, 1 nanometer or less.
Ultra thin metal silicide layers are particularly useful as Schottky barriers in infrared imaging arrays, generally referred to as IR-CCD's. Such devices usually include infrared detectors that generate charge in response to thermal radiation and also include readout registers in the form of charge coupled devices that scan the detectors and output video signals. These devices are formed on high resistivity, P type silicon substrates having high sheet resistance uniformity. The infrared detectors are Schottky barrier photodiodes comprising layers of metal silicides, such as platinum silicide or palladium silicide formed on the orientation, i.e. 100 or 111, of the substrate. In operation, these detectors respond to infrared radiation emitted by objects to be viewed and this radiation causes the photoexcitation of hole-electron pairs in the silicide layer followed by the internal photoemission of holes over the contact barrier. It is the electrons left on the silicide electrode layer that are read out by the charge coupled devices.
It is well established, for example see ADVANCES IN PLATINUM SILICIDE SCHOTTKY-BARRIER IR-CCD IMAGE SENSORS by Kosonocky et al. in SPIE, Vol. 225, pages 69-71 (1980), that the sensitivity to infrared radiation of metal silicide layers in Schottky barrier detectors increases as their thickness decreases. It is reported therein that a Schottky barrier layer formed from a 10 nm thick layer of deposited platinum has about five times the sensitivity of a similar layer formed from a 60 nm thick layer of deposited platinum. Until now, the problem has been how to form such ultra thin layers uniformly and reproducibly. The results of efforts to date to produce ultra thin metal silicide layers have been inconsistent in that the layers were not uniform in thickness and/or did not completely cover the substrate. The incidence of such nonuniformities has been shown to be related to the metal being used; e.g., there is a greater incidence of nonuniformities using palladium than platinum. In order to obtain complete coverage of the substrate, it has been necessary heretofore to increase the thickness of the metal layer. While this may resolve the problem of incomplete coverage, it usually aggravates the nonuniformity problem. A means of providing an ultra thin metal silicide layer which has neither of these disadvantages has been found in accordance with this invention.